A schematic cross-sectional diagram of a pump chamber and its periphery of a general liquid feed pump is shown in FIG. 4.
A pump body 18 and a pump head 8 are provided. Although not shown, a crosshead that reciprocates in one direction (the left/right direction in the drawing) by a cam mechanism is accommodated inside the pump body 18, and a plunger 3 is held at a tip end of the crosshead. The plunger 3 reciprocates in the axial direction according to the movement of the crosshead.
The pump head 8 is provided with a pump chamber 8a for storing suctioned liquid, and a suction port 8b and a discharge port 8c that are paths that communicate with the pump chamber 8a from the outside. A tip end of the plunger 3 penetrates a tip end of the pump body 18, and is inserted into the pump chamber 8a inside the pump head 8. Suction of liquid from the suction port 8b and discharge of liquid from the discharge port 8c are performed by a tip end portion of the plunger 3 sliding inside the pump chamber 8a. 
With such a liquid feed pump, a plunger seal 13 for preventing leakage of liquid from a gap between an inner wall of the pump chamber 8a and an outer circumference of the plunger 3 is attached to a portion of the pump chamber 8a into which the plunger 3 is to be inserted (for example, see Patent Document 1). The plunger seal 13 is a ring-shaped resin material having a penetration hole through which the plunger 3 is to penetrate.
The plunger seal 13 is sandwiched between the pump head 8 and the pump body 18 for holding the pump head 8. Also, a backup ring for improving the sealing performance of the plunger seal 13 is sometimes inserted at the back side of the plunger seal 13. In this case, the plunger seal 13 is sandwiched between the backup ring and the pump head. The backup ring is biased toward the plunger seal 13 by the pump body, and the plunger seal 13 is biased toward the pump head 8 by the backup ring.